Method of manufacturing β type silicon nitride whiskers

ABSTRACT

Silicon nitride whiskers having a long fiber length in which β type silicon nitride is well developed are produced by reacting a mixture of silica, carbon and cryolite in the specific molar ratio in a mixed gas atmosphere of N 2  and NH 3  by heating the mixture at a temperature of 1,250°-1,450° C. Kira of a ceramic industry waste may be used in place of silica and in this case N 2  alone is used as a nitriding atmosphere.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to a method of manufacturing β typesilicon nitride whiskers, and particularly to a method of manufacturingβ type silicon nitride whiskers by which well developed β type siliconnitride whiskers having a long fiber length can be easily produced at arelatively low temperature of 1,250°-1,450° C.

(2) Description of the Prior Art

In general, the application of β type silicon nitride whiskers has beeneagerly expected as a fiber reinforcement for a composite materialbecause of excellent high temperature strength and thermal shockresistance of silicon nitride.

As the prior techniques relating to the present invention, there are thefollowing methods, but they are quite different from the presentinvention for the reasons also stated below. Therefore, the inventioncannot easily be inferred therefrom. The invention is to provide amethod of commercially advantageously producing β type silicon nitridewhiskers.

(A) Japanese Patent Application Publication No. 12,320/1976 "A Method ofManufacturing Silicon Nitride"

This reference is similar to the present invention in view of additionof a fluoride to a mixture of silica and carbon and in the compositionthereof, but is different from the present invention in a way of addingthe fluoride. Further, this reference is completely silent about themolar ratio of silica, carbon and cryolite, and it also fails to referto the crystal type of the whiskers.

(B) Japanese Patent Application Publication No. 21,160/1975 "A Method ofManufacturing Fibrous Silicon Nitride Crystals"

This is quite different from the present invention in that silicon isused as the source of Si of silicon nitride.

(C) Japanese Patent Application Publication No. 4,480/1975 "A Method ofManufacturing Silicon Nitride Fibers"

This reference uses a metal additive as a catalyst of the nitridingreaction, and therefore it quite differs from the present invention.This reference is utterly silent about the crystal form of the obtainedwhiskers.

(D) Japanese Patent Application Publication No. 27,755/1974 "A Method ofManufacturing Silicon Nitride Whiskers"

This reference adds Cl component in N₂ atmosphere, and therefore itdiffers from the present invention. Further, it does not to the crystalform of the produced whiskers at all.

Briefly speaking, there have been conventionally known the followingmethods as the method of manufacturing the silicon nitride whisker.

(a) Direct nitriding of metallic silicon powder.

(b) Catalytic reaction of silicon halide with ammonia.

(c) In a silica reduction method, a fluoride or chlorine component isadded, and silica is reacted with N₂ gas.

According to the methods (a) and (b), merely powdery whiskers having ashort fiber length can be obtained. According to the method of (c),although the whiskers of a slightly longer fiber length can be obtained,the structure is mainly of α type when the treating temperature is arelatively low temperature of 1,300° to 1,400° C.

In the conventional techniques, a problem is the phase transition of Si₃N₄ in the case that the Si₃ N₄ whiskers are mixed into Si₃ N₄ powder,and are used as a reinforcement of the Si₃ N₄ sintered body. Si₃ N₄ hastwo crystal phase, α type and β type, which belong to the hexagonalsystem. The dimension of crystal lattices of both the crystal types isthat a_(o) =7.76 Å and c_(o) =5.62 Å in the case of the α type, whilea_(o) =7.61 Å and c_(o) =2.91 Å in the case of the β type. The dimensionof the crystal lattice in the c-axis direction is largely differentbetween the α type and the β type. β is a high temperature phase whichis changed from the α type at about 1,400°-1,600° C. The α type Si₃ N₄powder having a high purity and a high sinterability is suitable for theraw material of the Si₃ N₄ sintered body. This material is considered toimpart a high strength to the sintered body through phase transitionfrom the α type to the β type in the course of sintering. It is reportedthat when Si₃ N₄ whiskers abundant in the α type are added to such an αtype Si₃ N₄ powder and the mixture is sintered, the whiskers themselvesundergo the phase transition in the course of the sintering, so that thestrain is caused between matrices to lower the strength of the sinteredbody. It is considered that the influence of the phase transition of theSi₃ N₄ whiskers similarly occurs in the case of the reinforcing of asintered body other than Si₃ N₄. Thus, it has been long demanded todevelop a method of manufacturing Si₃ N₄ whiskers composed mainly of theβ type.

SUMMARY OF THE INVENTION

In view of the above circumstances, upon strenuously having madestudies, the present inventors have accomplished the invention. That is,it is therefore an object of the present invention to provide a methodof manufacturing a well developed β type Si₃ N₄ whiskers of a long fiberlength at a relatively low temperature, for instance, 1,250°-1,450° C.

It is another object of the present invention to provide a cheap methodof manufacturing a well developed β type Si₃ N₄ whiskers of a long fiberlength at a relatively low temperature, for instance, 1,250°-1,450° C.using Kira as industrial waste.

According to the method of manufacturing the β type silicon nitridewhiskers in the present invention, a mixture of silica, carbon andcryolite in a molar ratio of 1:(2˜10):(1/12˜1/4) is heated, at arelatively low temperature of 1,250°-1,450° C. in a mixture atmosphereof N₂ +NH₃, a molar ratio of NH₃ to N₂ being not higher than 1/5.

According to the second aspect of the present invention, there is aprovision of a method of cheaply manufacturing β type silicon nitridewhiskers having a long fiber length by heating a mixture of silicacontained in Kira, carbon, and cryolite in a molar ratio of1:(3˜13):(1/12˜1), at a temperature of 1,250°-1,450° C. in an N₂ gasatmosphere.

These and other objects, features and advantages of the presentinvention will be well appreciated upon reading of the followingdescription of the invention when taken in connection with the appendedclaims and the attached drawings with understanding that somemodifications, variations and changes could be easily made by theskilled in the art to which the invention pertains without departingfrom the spirit of the invention and the scope of the claims.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematically sectional view of a device used in carryingout a process according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The production reaction of silicon nitride (Si₃ N₄) whiskers accordingto a silica reduction process using N₂ gas is shown by the following twoformulae:

    SiO.sub.2 (solid)+C(solid)→SiO(gas)+CO(gas) . . .   (1)

    3SiO(gas)+2N.sub.2 (gas)+3CO(gas)→Si.sub.3 N.sub.4 (solid)+3CO.sub.2 (gas) . . .                                               (2)

The inventors have found that β type Si₃ N₄ whiskers can be manufacturedat a relatively low temperature by reacting silica with carbon in thepresence of cryolite in N₂ atmosphere to which a small amount of NH₃ isadded.

Since the reaction in the formula (1) is a solid reaction between SiO₂and C, the reaction rate is slow. When cryolite is present in thereaction between SiO₂ and C, however, SiO₂ is changed to a silicatefused body by the action of this flux, which results in the reactionbetween the liquid and the solid C and is represented by the followingformula (3) :

    SiO.sub.2 (silicate, liquid)+C(solid)→SiO(gas)+CO(gas) . . . (3)

The reaction rate of this reaction is far promoted as compared with theconventional solid reaction and the production of the SiO is extremelyactivated.

Meanwhile, when NH₃ is added to the reaction atmosphere, gaseous SiOproduced in the formula (3) reacts according to the following formula(4) due to a powerful reducing action of H₂ gas generated throughthermal decomposition of NH₃.

    SiO(gas)+H.sub.2 (gas)→Si(gas)+H.sub.2 O(gas) . . . (4)

The thus produced Si (gas) reacts with N₂ to produce crystal nuclei of βtype Si₃ N₄ according to the following formula (5):

    3Si(gas)+2N.sub.2 (gas)→Si.sub.3 N.sub.4 (solid) . . . (5)

It is in conformity with the ordinary crystal growth theory that thecrystal nuclei of β type Si₃ N₄ thus formed grow as the β type crystalseven under conditions where the α type crystals are produced accordingto the reaction (2).

Therefore, the production of the β type Si₃ N₄ whiskers proceedsaccording to the reactions (4), (5) and (2). It is a question why the βtype Si₃ N₄ is produced according to the reaction (5) although when asingle crystal of Si is heated and converted to Si vapor, which isreacted with N₂ gas, α type Si₃ N₄ is produced. This is because Si vaporobtained through reduction of SiO under heating is different in physicalproperties from that obtained by heating the Si single crystal. This maybe illustrated by the similar phenomenon in the case where a singlecrystal is produced from a fused body as another embodiment in which asingle crystal containing Fe^(II) cannot be obtained from a fused bodyin which the starting material is Fe^(II), while a single crystalcontaining Fe^(II) can be obtained from a fused body obtained byreducing a starting material of Fe^(III) at a high temperature intoFe^(II) and this is based on the difference of the physical propertiesof both Fe^(II). This is due to the difference in the activity and theion radius and in this case, in order to reduce the strain of thecrystals, the β type product having a smaller c_(o) value is produced.Further, in this case, the reaction (2) may independently take placedepending upon the reaction conditions to produce the α type productsimultaneously.

According to the invention, the mixing ratio of silica to carbon is 1:2to 1:10, preferably about 1:4 in a molar ratio. If the molar ratio ofcarbon is lower than the above range, the reaction (1) is difficult toproceed, while on the contrary, when it exceeds the above range, theamount of unreacted carbon residue increases, and the reaction yieldunfavorably lowers.

The mixing ratio of silica to cryolite cannot be definitively determinedbecause the synergistic effects of the cryolite and NH₃ added in thenitrogen atmosphere is remarkable, but is preferably about 1:1/12-1:1/4in a molar ratio of SiO₂ :Na₃ AlF₆. When the molar ratio of N₂ to NH₃ inthe mixed atmosphere (N₂ +NH₃) is 24:1, the ratio of SiO₂ to Na₃ AlF₆ ismore preferred to be 1:1/10-1:1/6. If the amount of cryolite is morethan the above range, the amount of fused silicate formed in thereaction is too much, so that the flowing movement of N₂ gas and SiO gasis interrupted. Thereby, the reactions (4) and (5) are difficult toproceed and SiC is unfavorably formed in the residue. Reversely, when itis less than the above range, the reaction (3) is retarded and theunreacted SiO₂ is unfavorably formed in the residue. The mixing ratio ofNH₃ to N₂ is preferably not higher than 1/5, more preferably not higherthan 1/12 in a molar ratio. If the mixing ratio exceeds 1/5, the partialpressure of N₂ in the mixing gas atmosphere is too lower and a largeamount of SiC is likely to be unfavorably produced in the residue. Inthe case of N₂ gas alone, α type Si₃ N₄ whiskers are likely to beproduced and no β type Si₃ N₄ whiskers are obtained.

As mentioned above, an appropriate amount of NH₃ and cryolite areindispensable for the present invention.

The reaction temperature is 1,250°-1,450° C. If the temperature is lowerthan 1,250° C., the reaction is difficult to proceed and the yield isextremely low. On the other hand, if it exceeds 1,450° C., no siliconnitride is produced and silicon carbide is produced instead. Thereaction temperature is preferably 1,300°-1,500° C.

The inventors have found that Kira of ceramic industry waste can be usedin place of silica which is the raw material in the above describedmethod.

The term "Kira" used herein means a residue obtained by elutriating araw loam for ceramics to remove clay and quartz and which are used for aceramic raw material. Kira has a particle distribution of about 5-40μand a chemical composition of around 80% of SiO₂, around 10% of Al₂ O₃,and slight amounts of others such as Fe₂ O₃, K₂ O, Na₂ O and consistsmainly of crystal, feldspar, kaolinite and mica. This Kira is so far lowin practical value and is produced in a large amount. Discharge ordiscard of such a large amount of Kira into rivers leads to publicnuisance, so that the disposal thereof now comes into a problem, andtherefore the investigation on the utilization of this industrial wastehas been strongly demanded.

Thus, the second aspect of the present invention is to provide a methodof cheaply manufacturing β type silicon nitride whiskers having a longfiber length by effectively using cheap Kira of the industrial wastewithout using the expansive silica source. Silica particles in Kira arehigh in the reactivity and contain Al₂ O₃, Fe₂ O₃, K₂ O, Na₂ O, etc. andare easily fused.

Thus, the second aspect of the present invention has been accomplishedby utilizing such properties and is to provide a method of cheaplymanufacturing β type silicon nitride whiskers having a long fiber lengthby heating a mixture of silica in Kira, carbon, and cryolite in a molarratio of 1:(3˜13):(1/12˜1), at a temperature of 1,250°-1,450° C. in anN₂ gas atmosphere.

The mixing ratio of silica contained in Kira to carbon is 1:3 to 1:13,preferably, around 1:8 in a molar ratio. As mentioned in the above firstprocess, if the molar ratio of carbon is lower than the above range, thereaction (1) is difficult to proceed, while on the contrary, when itexceeds the above range, the amount of unreacted carbon residueincreases, and the reaction yield unfavorably drops. The reason why themolar ratio of carbon is larger in this case than in the case wheresilica itself is used as a raw material is that a part of carbon isconsumed in the reduction reaction of impurity oxides.

The molar ratio of silica to cryolite is in a range of 1:(1/12˜1),preferably around 1:1/3. If the amount of cryolite is more than theabove range, the amount of the silicate fused body produced in thereaction becomes too much, so that the partial pressure of fluorine inthe gas phase becomes high and the partial pressure of SiO reverselybecomes too low, so that the efficiency lowers. Reversely, when cryoliteis less than the above range, the formation of the fused body isdifficult and the reaction rate becomes slow and the efficiency lowers.

The reason why the molar ratio of cryolite is larger in this case thanin the case where silica itself is used as the raw material is that apart of the cryolite is consumed in the reaction with the impurityoxides as in the case of carbon.

The reaction temperature is the same as in the case of using itself asmentioned above.

The present invention will be explained more in detail with reference toExamples in comparison with Comparative Examples. These Examples aremerely illustrative of the invention, and are never interpreted to limitthe scope of the invention.

EXAMPLES USING SILICA

Silicate anhydride (silica), activated carbon and cryolite each beingpowder passing through 100 meshes were dry-mixed with one another inmolar ratios as shown in the following Table to obtain a sample material5. 1 g of each of the mixed raw material samples 5 was charged into avessel 4 made of carbon as shown in FIG. 1, which was then placed into aprotection tube 3 made of mullite. This mullite tube was placed into anelectric furnace 1 using a furnace core tube 2. While a mixed gas 6 ofN₂ and NH₃ having a molar ratio as shown in the following Table wasintroduced into the furnace core tube 2 in a direction shown by thearrow in FIG. 1, the reaction was effected in the atmosphere of N₂ +NH₃at a temperature of 1,350° C. for 24 hours under heating. On the otherhand, while an exhaust gas 7 was exhausted in a direction shown by thearrow in FIG. 1. In this case, the flow rate of the introduced mixedatmosphere gas 6 was 50 cc/min, and the reaction was effected while thepressure of the interior of the furnace core tube 2 was kept at aboutatmospheric pressure. The silicon nitride whiskers produced on the innersurface of the protection tube 3 was observed by X-ray diffraction. Thecontents (weight %) of α phase and β phase are also shown in thefollowing Table 1.

                                      TABLE 1                                     __________________________________________________________________________               Compar-   Compar-               Compar-                                                                            Compar-   Compar-                        ative     ative                 ative                                                                              ative     ative                          Example                                                                            Example                                                                            Example                                                                             Example                                                                            Example                                                                            Example                                                                             Example                                                                            Example                                                                            Example                                                                            Example                        1    1    2     2    3    4     3    4    5    5                   __________________________________________________________________________    Raw   Silica                                                                             1    1    1     1    1    1     1    1    1    1                   material                                                                            Carbon                                                                             4    4    4     4    4    4     4    4    4    4  (molar Cryoli                                                              te 1/26 1/10 1/6                                                              31/6 1/6 1/6 1/6                                                              31/6 1/4 1/3        ratio)                                                                        Atmo- N.sub.2 :NH.sub.3                                                                  24:1 24:1 N.sub.2                                                                             48:1 24:1 17:1  3:1  1:1  24:1 24:1                phere (molar         alone                                                    gas   ratio)                                                                  Whisker                                                                             α type                                                                       50%  10%  40%   20%   5%  15%   β'-                                                                           β'-                                                                           10%  β'-                                                       cyalone                                                                            cyalone   cyalone                   β type                                                                        50%  90%  60%   80%  95%  85%   SiC  SiC  90%  SiC                                                            produced                                                                           produced  produced            __________________________________________________________________________

As apparent from the above Table, the amount of cryolite in the rawmaterial 5 in Comparative Examples 1 and 5 is outside the range in themethod according to the present invention, and these ComparativeExamples are unfavorable in that the content of the α type siliconnitride whiskers is high or β' cyalone (Si_(6-z) Al_(z) O_(z) N_(8-z):1≦z≦4), and SiC are produced.

In Comparative Examples 2 to 4, the molar ratio of N₂ +NH₃ in the mixedatmosphere gas 6 was outside the range of the present invention. Theyare no good in that the content of the α type silicon nitride whiskersis high, or β' cyalone and SiC are produced.

On the other hand, in any of Examples 1-5 of the method according to thepresent invention, well developed β type silicon nitride whiskers havinga high purity, a content of the β type silicon nitride of not lower than80%, a diameter of 0.2-1.5μ and a length of about 5 mm in the averagewere obtained. Namely, the conspicuous effects of the present inventioncan be observed in these Examples.

As mentioned above, according to the present invention, there isprovided a method of easily manufacturing the flexible and welldeveloped β type silicon nitride whiskers having a long fiber length ata relatively low temperature.

EXAMPLE USING KIRA

Powders of Kira, carbon and cryolite were dry-mixed in the molar ratiosshown in Tables 2 and 3. Kira is represented by SiO₂ contained therein.

1 g of the mixture raw material 5 in FIG. 1 was charged into a vessel 4,made of carbon, which was placed in a protection tube 3 made of mullite.This protection tube was then placed into an electric furnace 1 using amullite tube as a furnace core tube 2.

                  TABLE 2                                                         ______________________________________                                        Sample                   (1,350° C.)                                   No.         SiO.sub.2 :C:Na.sub.3 AlF.sub.6                                                            Yield (%)                                            ______________________________________                                        1           1:4:1/3       3.6                                                 2           1:5:1/3      10.7                                                 3           1:6:1/3      23.1                                                 4           1:7:1/3      22.8                                                 5           1:8:1/3      27.0                                                 6           1:10:1/3     19.8                                                 ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        Sample                   (1,350° C.)                                   No.         SiO.sub.2 :C:Na.sub.3 AlF.sub.6                                                            Yield (%)                                            ______________________________________                                        7            1:8:1/12     4.8                                                 8           1:8:1/6      18.3                                                 9           1:8:1/3      27.0                                                 10          1:8:1/1      11.2                                                 ______________________________________                                    

The mixture of silica in Kira, carbon and cryolite in a molar ratio of1:8:1/3 was prepared and reacted at various temperatures as shown in thefollowing Table 4 and the obtained yield is shown in said Table.

                  TABLE 4                                                         ______________________________________                                        Sample                    (1:8:1/3)                                           No.         Temperature (°C.)                                                                    Yield (%)                                           ______________________________________                                        11          1,250          5.2                                                12          1,300         14.3                                                13          1,350         27.0                                                14          1,400         13.2                                                15          1,450          7.4                                                ______________________________________                                    

Reaction was carried out under heating at a temperature of 1,350° C. for24 hours while flowing N₂ gas 6 into the furnace core tube 2 from thearrow direction in FIG. 1.

Meanwhile, the waste gas 7 was discharged as shown by the arrow inFIG. 1. N₂ gas was flowed at a flow rate of 75 cc/min under atmosphericpressure. The silicon nitride whiskers formed in the inner surface ofthe protection tube 3 was examined through X-ray diffraction, and it wasrevealed that they were composed substantially of β phase containing aslight amount of α phase.

As mentioned above, according to the present invention, the β typesilicon nitride whiskers can be manufacture at a high yield by using acheap Kira as an industrial waste.

What is claimed is:
 1. A method of manufacturing β type silicon nitridewhiskers, which comprises preparing a mixture of silica, carbon, andcryolite in a molar ratio of 1:(2˜10):(1/12˜1/4), reacting said mixturein a mixed gas atmosphere of N₂ and NH₃ obtained by adding NH₃ to N₂, ina molar ratio of not higher than 1/5 based on N₂ by heating the mixtureat a temperature of 1,250°-1,450° C., said NH₃ being present in anamount sufficient to effect a higher content of β type silicon nitridethan that produced by said method in the absence of NH₃.
 2. The methodaccording to claim 1, wherein the molar ratio of silica to carbon (SiO₂:C) is about 1:4.
 3. The method according to claim 1, wherein the molarratio of silica to cryolite (SiO₂ :Na₃ AlF₆) is 1:1/10˜1/6.
 4. Themethod according to claim 1, wherein the molar ratio of NH₃ to N₂ is nothigher than 1/12.
 5. The method according to claim 1, wherein thereaction temperature is 1,300°-1,400° C.
 6. A method of manufacturing βtype silicon nitride whiskers by using Kira as a silica source, whichcomprises preparing a mixture of Kira, carbon and cryolite in a molarratio of silica contained in Kira, carbon, and cryolite of1:(3˜13):(1/12˜1), reacting the mixture in an atmosphere of N₂ gas byheating the mixture at a temperature of 1,250°-1,450° C.
 7. The methodaccording to claim 6, wherein the molar ratio of silica to carbon isapproximately 1:8.
 8. The method according to claim 6, wherein the molarratio of silica to cryolite is approximately 1:1/3.
 9. The methodaccording to claim 6, wherein the reaction temperature is approximately1,350° C.